Regarding an optical communication it becomes extremely important to make use of an optical amplifier that generates a noise to be lower and that brings a gain to be higher in order to perform a compensation of a transmission loss in an optical fiber or of an insertion loss of an optical component, such as an arrayed waveguide grating (AWG) or the like. And then a semiconductor optical amplifier (SOA) as an electric current excitation type is the optical amplifier which is smaller in package and at a low price as well, because it is not necessary for the same to make use of a pump laser, that is different from an Er-doped fiber amplifier (EDFA). And then in recent years in particular, a semiconductor optical amplifier is paid attention extremely because the same has a property of which is smaller in package, from a point of view of such as that it is possible to integrate to be a fine optical circuit, such as the AWG or the like. Moreover, at the beginning of the development of the semiconductor optical amplifier the same is inferior in the properties of a saturation power output and a noise figure (NF) with comparing to each of that of the EDFA respectively. However, the development has been progressed in recent years, and then there becomes to be reported a semiconductor optical amplifier that has the property to be not inferior to that of the EDFA from the point of view of the saturation power output and the noise figure with comparing to each of that of the EDFA respectively (refer to the following nonpatent document 1).
Here, FIG. 8 is a cross sectional view for exemplary showing one example of a structure of a conventional semiconductor optical amplifier that functions as an optical amplifier. And then such a semiconductor optical amplifier (200) comprises a structure in which a cladding layer at a lower part (23) as an (n) type that functions as a buffer layer as well, an active layer (24) that is formed of a semiconductor and cladding layers at an upper part (25a) and (25b) as a (p) type for both are grown on a substrate (22) as an (n) type on which an electrode at an (n) side (21) is formed at a rear surface, that are shown in FIG. 8. Moreover, from a part of the substrate (22) to the cladding layers at the upper side as the (25a) and the (25b) becomes to be a mesa structure. And then both sides of the structure is implanted with making use of each of electric current blocking layers (30) and another (30) that individually are comprised of an electric current blocking layer at a lower part (30a) as a (p) type and an electric current blocking layer at an upper part (30b) as an (n) type, respectively. Further, a cladding layer at an upper part (25c) as a (p) type and a contact layer (26) as a (p) type are grown on the cladding layer at the upper part (25b) and to each of the electric current blocking layer as the (30) and the other (30). Still further, a protective coat film layer (27) and an electrode at a (p) side (28) are formed on the contact layer (26), and then an electrode at an outside (29) is formed thereon as well. Still further, in accordance with the semiconductor optical amplifier (200) two of end faces are formed in approximately parallel to the paper in a direction vertical to the paper. Furthermore, an anti-reflection coated film layer is formed on the two of the end faces.
And then the semiconductor optical amplifier (200) becomes to function as the following. In the first instance, a voltage is applied to between the electrode at the (n) side (21) and the electrode at the (p) side (28), and then an electric current is injected into the semiconductor active layer (24) so as to be an excited state. Moreover, the electric current is injected efficiently into the semiconductor active layer (24) by making use of the electric current blocking layer (30). And then after the semiconductor active layer (24) becoming the excited state in such a manner a light is input from one end face, that is to be performed an amplification, such as a light that has a wave length in a band of 1.55 μm or the like that is made use for the optical communication. And hence the light becomes to be amplified due to a function of a stimulated emission with being wave guided by making use of the semiconductor active layer (24). And then as a result a light that is amplified becomes to be output from the other end face.
Further, in accordance with the semiconductor optical amplifier (200) each of the end faces becomes to have a predetermined coefficient of reflection in a case where the anti-reflection coated film layer is not formed at the two of the end faces. And then therefore it is able to form an optical cavity as a Fabry-Perot type with making use of the two of the above mentioned end faces, and then it is able to make use of the device as a semiconductor laser equipment. Furthermore, it is able to make use of the semiconductor optical amplifier (200) as the semiconductor laser equipment in the same way even in a case where a reflection coated film layer is formed that has a desired coefficient of reflection in the place of the anti-reflection coated film layer.    [Non Patent Document 1] K. Morito et al., “A Broad-Band MQW Semiconductor Optical Amplifier With High Saturation Output Power and Low Noise Figure”, IEEE Photonics Technol. Lett., Vol. 17, No. 5, pp. 974-976, May 2005.